Vaccination against influenza is the mainstay of prevention efforts and was initially targeted at older individuals and those with high risk of complications. However, children are at the greatest risk of illness from influenza with infection rates averaging higher than any other age group and have been identified as predominant vectors in the household spread of influenza. Therefore, besides directly benefiting the pediatric population, the routine vaccination of children has potential to substantially reduce population-wide transmission. For 2006- 2007 influenza season, universal vaccination recommendations will include three and four year olds. This vaccine policy shift creates conditions to answer a critical question: will the new vaccine recommendation result in a lower burden of influenza across both pediatric and adult age groups? Beyond the effect of age group, other population-level demographic characteristics such as population density, poverty and household size may be important determinants of influenza spread and thus high priority targets. Although there is some empirical evidence on the relationship between geography and influenza across large scales, there is surprisingly little empirical information on the factors that influence local spread. Documenting empirical spatial patterns of influenza epidemics represents a first step toward understanding underlying mechanisms that drive epidemic fluctuations. The overall goal of this research is to use empirical evidence from both national and local surveillance data to evaluate high value targets for influenza control and prevention. The underlying factors controlling patterns of illness across space and time, on both local and national levels will be modeled to answer two critical questions: (1) Will the new recommendations reduce morbidity in the targeted age group (3-4 years) as well as overall morbidity and mortality across the population? (2) Are there other at-risk populations that would make important targets for surveillance and control based on their susceptibility and capacity to spread to virus? Aim 1 will rely on a natural experiment to empirically assess the role of vaccination of preschoolers. Aim 2 will involve exploratory work to model local geographic spread of influenza and identify important census-level covariates that influence the timing and magnitude of disease transmission. The proposed study leverages unique surveillance datasets to evaluate the current policy as well as to address possible extensions to it. For each aim, cross-sectional time analysis will be performed at two scales: state- wide, and national. The project will exploit rich and unique data sources including, pediatric hospitalizations, emergency visits and mortality, all collected prospectively. Statistical models will be based on these historical data to determine whether new policy influences the observed transmission patterns population (Aim 1), and whether targeting other at-risk populations would provide additional reduction of community-wide illness (Aim 2). The findings of this study will allow decision makers to formulate health policy regarding resource allocation and vaccination strategies for both inter-pandemic and pandemic influenza. The overall goal of this research is to use empirical evidence from both national and local surveillance data to evaluate high value target populations for influenza control and prevention. The findings of the proposed study will allow decision makers to formulate health policy regarding resource allocation and optimal vaccination strategies for both epidemic and pandemic influenza. In particular, the study will evaluate whether a recommendation by the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices to vaccinate three and four year olds will result in a lower burden of population-wide influenza burden and whether new policy should also focus on other at-risk populations. [unreadable] [unreadable] [unreadable]